Recently, there has been a significant increase in alternative fuel vehicles including those that use compressed natural gas (CNG) or gaseous hydrogen (H2) as a fuel source. However, unlike liquid fuel, which consistently occupies about the same volume of fuel across a broad range of conditions, the amount of gaseous fuel inside a storage vessel depends on the pressure and temperature of the gas. The problem of “under-filling” is a phenomenon whereby a vessel is filled with a less-than-desired amount of gaseous fuel. Typically, during a gas-filling process, the gas temperature rises. This temperature rise reduces the density of the gas and packs fewer gas molecules in the vessel than what would be possible had the temperature remained constant. As the temperature of the gas in the cylinder equilibrates with the environment after the filling process is complete, the user ends up with less amount of gas in the vessel than what the rated pressure and initial temperature at steady-state conditions would have allowed. This essentially results in an under-filled vessel relative to its rated specification.
Under-filling of gaseous fuel during a filling process causes reduced driving range of vehicles that use CNG, H2 or any other suitable gas as a fuel. Typically, compensation for the temperature rise is accomplished in the fuelling station dispenser by transiently over-pressurizing the vessel. The amount of over-pressurization is dictated by the capabilities of the filling station, the pressure differential between the dispenser of the filling station and the vessel, the ambient temperature and the design capacity of the vessel to handle pressures higher than its rated designation. This over-pressurization of a fuel storage vessel can be extremely dangerous if the vessels are not designed adequately and can reduce the cycle life of the vessel significantly. Currently there are no means to compensate for the under-filling problem by thermal management of the heat generated during the filling process in-situ of the vessel.
Methods or devices available that can reduce the amount of temperature increase during a gas-fill process, in particular during a fast-fill process, are limited to active cooling or refrigeration of the gas at the filling port. This process is energy intensive, requires expensive equipment and adds additional cost to the filling process and consequently the fuel filled. With an increase in alternative fuel vehicles that use CNG or hydrogen as a fuel source, there is an increasing need for enabling the fast-filling process without a significant increase in temperature, and/or a need for over-pressurization or active refrigeration of the filling gas.